Transcript Part 2

Chapter 2
Communications
Systems Modeling
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Contents
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Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
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Contents
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•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
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Layered Network Models
• The problem:
Complexity of networked systems
• The solution:
Decomposition into independent subsystems
• Layered network model
– Introduced by IBM (SNA, 1971)
– Standardized by ITU (X200) and ISO (7498)
– Method universally adopted
– Standards not really obeyed in practice
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Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
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A diplomatic negotiation
Political arguments
Service Access Points
Translation
Cryptography
Transmission
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A diplomatic negotiation
Political arguments
Service Access Points
Single language messages
Cryptography
Transmission
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A diplomatic negotiation
Political arguments
Service Access Points
Single language messages
Encrypted messages
Transmission
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A diplomatic negotiation
Political arguments
Service Access Points
Single language messages
Encrypted messages
Electromagnetic waves
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Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
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Layered Model Definitions
Level (n+1)
entity
Virtual level n+1 communication path
using a level n+1 protocol
Level (n+1)
entity
Physical Communication Path
Level n
Service Access Points
Level (n)
entity
Virtual level n communication path
using a level n protocol
Level (n)
entity
Level n Addresses
Level (n-1) Service Provider
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Layered Model Definitions
Example: The Telephone Service
Virtual Interpersonal communication path
Physical Communication Path
Telephone
Service Access Points
Telephone Numbers
Telephone Network
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Layered Model Definitions
Protocol Data Units
Level (n+1)
entity
(n+1)PDU
Level (n+1)
entity
PDU encapsulation
Level (n)
entity
(n)PDU
Level (n)
entity
Level (n-1) Service Provider
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Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
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Connection Oriented
Communications
Typical example:
A telephone conversation
3 step process:
1. Establish connection
using the signaling protocol
2. Use connection
using an informal protocol
being billed for duration of usage
3. Terminate connection
using the signaling protocol
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Connection Oriented
Communications
A temporary communication channel
is built through the telephone network
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Connection Oriented
Communications
• Network has a state : the list of active connections
• Network can guarantee Quality of Service
– Reservation of transmission capacity at connection
establishment
– Monitoring of data transfer and correction of
transmission errors by retransmission
• Network is fragile : state can be lost.
• Application domain :
– Very efficient for transferring high volumes of data
– Mandatory if stringent Quality of Service requirements
– Poor initial response times due to connection setup
– Inapplicable if nodes have high failure rate.
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Connectionless
Communications
Typical example:
The postal service
Letters, with an address and a stamp,
are individually carried from a
Post office box to a personal mailbox
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Connectionless
Communications
No connections are needed to carry mail
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Connectionless
Communications
• The network is stateless.
• No guaranteed Quality of Service.
– No reservation of transmission capacity
– No error correction by retransmission
• Network is robust :
– No state stored in any node
– Each node operates almost independently from
others
• Application domain :
– Whenever single message response times are
critical
– Whenever nodes are unreliable
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Connection Policy
Layer independance
Level (n+1)
entity
Connectionless protocol
Level (n+1)
entity
Connection oriented protocol
Level (n)
entity
Level (n)
entity
Level (n-1) Service Provider
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Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
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A two layers model.
Networks Layer
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Interoperability
Connectivity
Applications Layer
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A three layers model.
Networks Layer
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Connectivity
Internet & Transport Layer
Interoperability
Applications Layer
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Internet & Transport Layers
Applications Layer
Transport Layer
Internet Layer
Networks Layer
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Internet & Transport Layers
Example
WWW
Transport Layer
TCP
TCP
Internet Layer
IP
SKYNET
IP
UUNET
Networks Layer
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Introduced concepts
• Layered communications system
– protocol = horizontal convention
– Service access point & address
• Connection Oriented vs. Connectionless Communications
• Applications and networks layers
• Connectivity :
– service offered by lower layers
• Interoperability :
– full communications services
• Internet & Transport layers
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Bibliography
To know More about network modeling
Fred Halsall
Data Communications,
Computer Networks
and Open Systems
Fourth Edition
Addison-Wesley 1995.
ISBN 0-201-42293-X
Recommended for this chapter
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